GNEI9SIC ROCKS OFF THE LIZARD. 311 



with a S.W. dip. This dyke is thickly studded with crystals of 

 felspar, and it branches and cuts the gneiss in an intricate manner. 

 The original dyke appears to have been about 18 inches wide, and 

 by jointing out to have caused a cleft on the east side of the rock. 



Pig. 3. — Portion of Basic Dijhe in the " Man-of- War " Bocks, 

 traversed hy veins of gneiss. 



The shaded part is the dyke. 



The high isolated mass of rock immediately south-west of the 

 mainland is the " Quadrant." When the sea is calm this island is 

 easily climbed, and the relations of the two basic porphyritic dykes 

 which traverse it can be well studied, free from the barnacles which 

 completely cover most of the other dykes. Approaching it from the 

 south-east the " Quadrant " appears, as roughly sketched in fig. 4, with 

 a dyke cutting the middle, faulted and almost perpendicular. When 

 seen from the north-north-west the rock appears somewhat as in fig. 5. 

 Both dykes are seen here, each about two feet wide, running E.N.E. 

 and W.S.W. with an average S.S.E. dip of 50°. The northern 

 dyke is a dark basic rock studded thickly with small crystals* of 

 felspar. In some places it weathers a glossy black ; in other places 

 :.he crystals project outside the weathered surface as if peppered on 

 the rock. On the north-east it is abruptly cut off by an open fault 

 or cleft (a, fig. 5). The southern dyke is seen to traverse the 

 highest point in the centre of the island. At the extreme top it 

 appears to be conformable with its gneissic surroundings and to dip 

 at a very high angle towards the east-north-east. It weathers dark 

 grey at this point with crowded projecting crystals, and is itself 

 traversed by a gneissic band I5 inch wide at 6, fig. 5. This dyke 



